It is desirable to have a steam sterilizable, clear, water-white plastic syringe which is cheap enough to be expendible as a throw away after a single use. To date this has not been achieved. It is known to make syringes from 4-methylpentene-1 polymer (available commercially as TPX). However, this polymer is quite expensive, the selling price for the polymer alone recently being $1.67 a pound.
In the manufacture of syringes for the injection of pharmaceutical products glass has been the preferred product. This is based on factors such as clarity, modest price and steam sterilizability. In spite of many advantages glass has the disadvantage of relatively poor impact resistance and shattering when it is broken.
A number of plastics have been investigated as a potential replacement for glass in the manufacture of syringes to circumvent the problem of breakage. Polystyrene has acceptable clarity, color, and price, but it cannot be steam sterilized. Polycarbonate has acceptable clarity, color and can be steam sterilized but is high priced as is 4-methylpentene-1 polymer mentioned above. Polypropylene can be steam sterilized and is modestly priced but has poor clarity.
Walling et al., J. Amer. Chem. Soc. 70 (1948) 1543-1544 refer to copolymerization of alpha methyl styrene with methyl methacrylate. Reactivity ratios for the two monomers were determined. The polymerization was carried out at 60.degree. C. in sealed tubes with benzoyl peroxide catalyst.
Ham U.S. Pat. No. 3,072,622 shows making copolymers of alpha methyl styrene and methyl methacrylate. While Ham claims reacting 10 to 40 parts by weight of alpha methyl styrene with 60 to 90 parts by weight of methyl methacrylate, in Examples I and II runs 11 and 12, he also discloses using 50 parts of alpha methyl styrene and 50 parts of methyl methacrylate. Ham on col. 2 lines 26-42 refers to Walling and points out that the Walling procedure gives low molecular weight polymers which are brittle and have low heat resistance which would have no utility as a plastic. Ham developed a process to overcome this and his copolymers are stated to be useful for making plastic articles by extruding, molding and casting. Sheets, rods, tubes, lenses and massive pieces can be made according to Ham. The products are stated to be useful in a manner similar to polymethyl methacrylate. The products of Ham are stated to be water white and to have high heat distortion temperatures, e.g., 140.degree.-145.degree. C. in Example II run 11 Table II.
Ham polymerizes in the absence of a solvent or water. While he only has a peroxide catalyst present in his examples he mentions that the polymerization can be improved by employing an organic disulfide or a mercaptan such as tertiary dodecyl mercaptan. Ham is assigned to the J. T. Baker Chemical Company.
Vandegaer U.S. Pat. No. 3,134,723 also assigned to J. T. Baker Chemical Company is directed to an improved process of copolymerizing 15 to 35 parts by weight of alpha methyl styrene with 65 to 85 parts by weight of methyl methacrylate. The improved technique is stated to reduce the time required for the reaction. The examples show various polymerization conditions including the use of tertiary dodecyl mercaptan as a polymerization regulator or stabilizer. Both peroxides and azo compounds are employed as catalysts. The reaction is carried out without the use of a solvent or water.
The copolymers are stated to have remarkable resistance to boiling water without losing their water-white optical clarity or shape or changing in physical dimensions. No mention is made, however, of resistance to steam sterilization at 250.degree. F. (about 121.degree. C.) The present inventors have tested commercial samples of alpha methyl styrene-methyl methacrylate copolymers made by the J. T. Baker Chemical Company as well as the copolymer of alpha methyl styrene and methyl methacrylate available commercially as Plexiglass II and found that after injection molding followed by steam sterilization at 250.degree. F. they were distorted and became opaque. In some instances shrinkage was as much as 30%.